This invention relates in general to thin-film solid state devices and more particularly to a method for making thin-film devices comprising semiconductive glass compositions.
Recently, semiconducting devices have been developed which are made from solid substances that are glassy rather than crystalline which nevertheless may be employed to control the flow of electric current. An important example of such a device is the Ovonic switch developed by Stanford R. Ovshinsky. His switch is a threshold device comprising a two-terminal component which may have two states in an electrical circuit, namely, an almost non-conducting state and a conducting state. The device is usually in the nearly non-conducting state, i.e., off; but when the voltage across the device reaches a certain threshold value, it goes to the conducting state, i.e., switches on. On the removal of the applied voltage, the off state is immediately restored.
Among the many types of non-crystalline materials which are being investigated for such devices are amorphous oxides (including oxides of the vanadium, tungsten, phosphorous, germanium and silicon) and chalcogenide glasses, which may be regarded as inorganic polymers. The term chalcogenic is applied to any of the elements in Group VIa of the periodic table: oxygen, sulphur, selenium, and tellurium. The chalcogenide glasses include binary systems (for example, germanium-tellurium), ternary systems (various three-component mixtures of germanium, arsenic, tellurium, silicon, selenium zinc, and cadmium) and quarternary systems composed of the same elements.
Semiconducting glasses of specific compositions have been shown by Shanefield in U.S. Pat. No. 3,448,425 to exhibit current controlled negative differential resistance (CNDR), rather than threshold switching (TS). Shanefield found that a semiconductive glass composition of the ternary group arsenic-tellurium-sellenium may have a voltage-current characteristic initially exhibiting relatively high resistance. As the applied voltage is increased, a form of "breakover" occurs in which the voltage-current characteristic exhibits negative incremental resistance. If after breakover has occurred and the applied voltage is maintained at a predetermined sustaining value, the glass exhibits relatively low electrical resistance. The glass is stable only in its high resistance state, and reverts to this state when the applied voltage is removed. These devices thus have a region of negative differential resistance which is suitable for the construction of practical oscillators, amplifiers and other devices employing negative resistance effects.
Fabrication of these thin-film devices has proven to be a delicate task, particularly in providing the critical registration requirements for a CNDR device. A fabrication process is taught herein which assures a high yield of such devices with improved reliability.
It is yet another object of the invention to fabricate thin-film devices at lower production costs.
Other objects of the invention will be evident from the description hereinafter presented.